This invention relates to a process for preparing polymer grade terephthalic acid from the disodium terephthalate contained in waste water from weight-reduction process in a polyester textile dyeing complex.
In general, in the prior art, disodium terephthalate contained in alkali waste water was discharged in a weight reduction-processing stage which is designed to give the polyester textile. A silky property, as well as to improve the dyeing capability by treating the polyester textile with sodium hydroxide or another basic aqueous solution at high temperature, and thereby causing a part of the textile to become depolymerized.
That alkali treatment technology for a polyester textile, causes the formation of the disodium terephthalate waste water. That alkali treatment technology was developed by Imperial Chemical Industries(England) in 1952, and it was well known that the weight of polyester could be reduced 15%-30% by treating the polyester textile with a sodium hydroxide solution. [M. N. Chapatwala, "Weight Reduction of Polyester" Colourage Dec. 1986].
Korea is one of the leading textile production countries in the world. Accordingly, in the past, a considerable amount of sodium terephthalate contained in waste water has been discharged because a large amount of polyester textile is treated by weight reduction process and this waste water that has been generated as a result, has been treated by a conventional method without a recovery of terephthalic acid from the waste water.
Therefore, instead of simply giving good properties to the polyester textile practicing the conventional weight reduction process not only has caused a loss of terephthalic acid, but also has caused severe environmental problems, when the disodium terephthalate contained in alkali waste water has been discharged directly into the environment.
Accordingly, the recovery of terephthalic acid from alkali waste water shows great concern, both as to re-utilization of terephthalic acid and the reduction of the load in treating, waste water.
The conventional weight-reduction process for polyester textile includes treating the polyester textile on about 2-12% sodium hydroxide solution at 60.degree. C.-130.degree. C. for a specified time. By using such a process, one can provide the polyester textile with a desirable property in which the surface of polyester textile is partly decomposed.
If the polyester textile, is treated with sodium hydroxide solution to reduce weight, a large amount of hydrophilic group is formed on the surface of the textile and some physical properties of the textile, such as the flexibility, absorption property, anti-electrification property, anti-contamination property, and so on, can be enhanced. By using the prior art method this changes in the physical properties exhibits similar touch and function to those of the natural fiber, silk.
In the course of conducting the alkali weight reduction process, disodium terephthalate and ethylene glycol are formed by depolymerization of the polyester on the surface of textile mixed with water for alkali waste water. This waste water should be treated to prevent pollution, but such treatment conventionally requires a large amount of investment for anti-pollution facilities or acids, such as the sulfuric acid which is used for neutralization of the waste water.
Recently, various methods relating to the recovery of terephthalic acid from the disodium terephthalate contained in water and on purifying crude terephthalic acid are reported. These related technologies are as follows:
In Japanese Patent Laid Open Publication No. 56-40,640, Crude terephthalic acid (16.6 parts) was dissolved in 2N-NaOH(150 parts), and 5% active chlorine contained NaOCl solution (8.25 parts) and 3.5% H.sub.2 O.sub.2 were added thereto, and the resulting solution was stirred for 60 minutes at 80.degree. C. to get the terephthalic acid having 90% of transmittance at 340 nm.
In Japanese Patent Laid Open Publication No. 46-19,619, crude terephthalic acid was dissolved in aqueous ammonia solution to recrystallize as ammonium salt and the resulting ammonium salt was treated with KMnO.sub.4 followed by passing the resulting aqueous solution through active charcoal, to thereby obtain a quantity of purified terephthalic acid.
In Japanese Patent Laid Open Publication No. 56-83,443, a solution containing terephthalic acid (150 parts) was washed with 2N-NaOH solution (1000 parts) having and water (150 parts), added KMnO.sub.4 (12 parts), reacted for 2 hrs at 80.degree. C. and treated with powdered active charcoal for 2 hrs at 80.degree. C. to thereby obtain terephthalic acid having 32% of transmittance. The above product was treated with 10% acetic acid solution (100 parts) for 1.5 hrs at 230.degree. C., and yielded terephthalic acid having 93% transmittance. (If the sample was not treated with KMnO.sub.4, the transmittance was 1%).
In Japanese Patent Laid Open Publication No. 56-113,738, crude terephthalic acid (150 parts) was dissolved in 2N-KOH (15 parts), water (500 parts) added thereto, and the reaction solution was stirred for 2 hrs at 250.degree. C., under a pressure of 20 atm. The resulting solution was cooled, KMnO.sub.4 (4.5 parts) was added thereto, at 90.degree. C., and reacted for 2 hrs. The solution was filtered to remove MnO.sub.2 and precipitated with sulfuric acid, and yielded terephthalic acid having 16% transmittance. The recovered terephthalic acid was treated with acetic acid-water (9:1) solution (100 parts) for 1.5 hrs at 230.degree. C. cooled, and filtered to thereby provide terephthalic acid having 92% transmittance.
In Japanese Patent Laid Open Publication No. 56-103,136, crude terephthalic acid (5.56 g) was hydrogenated in the presence of active charcoal 2%(1% ruthenium content) and active charcoal (1.0 wt % carbon) to obtain terephthalic acid having 62.9% transmittance.
In French Patent No. 1,592,084, crude terephthalic acid was dissolved in NaOH solution, and the pH of the solution was controlled to 9.0. That pH-controlled solution was heated to 50.degree. C.-60.degree. C. and sodium chloride was added thereto. After 30 min., the solution was filtered, and sulfuric acid was added to it in order to obtain terephthalic acid.
In Japanese Patent Laid Open Publication No. 45-11,293, crude terephthalic acid was dissolved in ammonia water, oxidized with KMnO.sub.4, and 8-hydroquinone was added thereto. The resulting solution was extracted with hydrochloric acid to obtain terephthalic acid.
In U.S. Pat. No. 3,849,489, crude terephthalic acid was dissolved in ammonia water, and heated to 290.degree. C. to recover terephthalic acid. The recovered terephthalic acid was dissolved again in ammonia water, and oxidized with MnO.sub.2. After 30 min., the solution was filtered, passed through active charcoal heated to 290.degree. C. for deammoniation, and terephthalic acid thereby obtained.
According to the reported methods, the crude terephthalic acid was purified by dissolving or washing with alkali solution such as NaOH or KOH, by treating at high temperature and high pressure using active charcoal for adsorption, or by treating with oxidizing agents or reducing agents such as MnO.sub.2, KMnO.sub.4 or hydrogen gas. The alkali transmittance was measured to identify the purity of the finally obtained terephthalic acid, but the values were always less than 93%.
These reported methods have also several problems, i.e., they are economically unfavorable because they use chemicals as oxidizing agents, and it requires long time to complete the processes.
In addition, the purity was identified only by alkali transmittance, and there can be come problems in testing the other physical properties.
Besides the above problems, terephthalic acid which can be reused in the polymerization reactions, should have more than 95% of alkali transmittance, so the terephthalic acid obtained by the reported methods was not sufficient to use as a polymer grade terephthalic acid.
Especially, it was regarded as a very difficult technology to get the sufficiently purified terephthalic acid as for the polymer grade, which can be reused in the polymerization reactions, from the recovered waste water, and the solution of this problem has importance not only in regard to the treatment of waste water for abating the environmental problem but also the recycling benefit of permitting utilization of terephthalic acid as a polymer grade.